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The lag phase, the specific growth rate, the minimum a [subscript w]
required for growth and the temperature characteristic of
Fsendomonas fluorescens, Brochothrix thermosphacta, Salmonella
typhlmurium, Streptococcus faecalis and, Staphylococcus aureus
have been studied in liquid model media under controlled temperature
and a [subscript w] conditions. The results show that the lag phase increases
and the specific growth rates decreases when the temperature is
lowered or the a [subscript w] is reduced. The minimum a [subscript w] required for growth
increases when the incubation temperature is lowered. At
refrigeration temperature the minimum a [subscript w] for growth of mesophiles
is higher than that of psychrotrophs. The temperature characteristic (representing the activation energy for growth) of psychrotrophs is lower than that of mesophiles, and therefore mesophiles are more
sensitive to temperature changes. All of these growth parameters
depend on the a [subscript w] controlling solute. The solute effect can be
traced to the ability of the solute to penetrate into the cell and
can be explained by the osmoregulatory mechanism. In general, it was
found that the effect of the solute on these parameters had the order
glycerol<NaCl<sucrose.
A comparison of three models for the effect of temperature on
growth rate (linear, square-root, and Arrhenius) showed that the
linear model had both the advantage of simplicity and accuracy. In
general, the Arrhenius model showed the poorest fit to experimental
data.
A predictive method was developed to assess the microbial
stability of liquid model systems exposed to fluctuating
temperatures. The strategy was to predict lag time and growth rate
by different regression equations, and to estimate microbial growth
using an integral function of the accumulated time. A lag time
predictive model was developed using a linear relationship between
lag time and reciprocal growth rate. A linear equation was used to
predict growth rate as a function of temperature. The model was used
to predict the growth of B. thermosphacta in liquid model media
with low a [subscript w] and exposed to fluctuating temperatures. The
predictions were acceptable at a 95% confidence level. This method
will be used in future studies to predict the microbial stability of
refrigerated foods exposed to temperature abuse.